It's practice, practice and more practice with experience. And yes all those gizmos just don't work.
One of the easiest ways to caulk a tub with good results is to use painters tape. With every clean surfaces run tape about an 1/8 of an inch on the wall and tub surface, fill the gap with caulking and smooth it out but leave a then level over the tapes edge. As the caulking starts to skin over, about 10-15 minutes, then remove the tape. I used this method when using silicone with frameless shower glass. There are videos on this method.
The thickness has a lot to do with the gap needed to fill. What happens a lot with tubs is if it is on a wooden floor, seasonal humidity change and floor settling will change the gap size and caulking may pull away. On a concrete slab it never moves. Two things for a good bead is cut the tube end as small as possible and second is how much to feed. I always use a gun to apply long beads of caulk and for a tub I try to make the bead as vertical as possible. It not possible to get the corner perfect so a wet finger is usually needed to smooth out. A thin bead it would not show much if it's not perfect. It's why I like siliconized caulk, if you mess up you can wipe it off with a wet rag and start over.
As always, having a clean surface to caulk on is the most important. If old caulking is being remove having a package of razor blades to get all the old stuff out. Rub it down with alcohol to remove any crude that might remain. If the gap is more than a 1/4" do it in stages. Fill the gap and with a putty knife or other tool push the caulking in but not to the tile edge and further in on the tub surface You want a little reveal at the tile for the second coat of caulking to do the next day. One little trick to learn is when doing a bathtub rim is point the nozzle of the caulking tube almost horizontal, about 75-80 degrees to the wall, not 45 degrees down from the top. You want it to go just past the vertical line of the tile on the tub surface. You don't want to be too concave because the caulk gets very thin at the edge and it then pulls away or lifts off overtime. This is where using painter tape comes in handy.